Fragments of calmodulin produced by limited trypsin hydrolysis can stimulate the adenylate cyclase of Bordetella pertussis spheroplast membranes in a Ca2+- dependent or Ca2+-independent manner with different potencies. The carboxy terminal fragment (residues 78-148) has high potency, but the cyclase is also stimulated by tryptic fragments 1-77, 1-90 and 107-148. Although very much less potent than calmodulin or fragment 78-148, the activation is not due to contamination by calmodulin but contamination of these fragments by fragment 78-148 is difficult to rule out for lack of an independent biological test. The high affinity Ca2+ binding sites probably reside in the carboxy-terminal fragment and the enhancement of the potency produced by Ca2+ ions is probably caused by these sites.